Pseudomonas aeruginosa is one of the main causitive factors of opportunist infections. Lipopolysaccharide isolated from P. aeruginosa AK1401 (LPS-defective rough mutant) was separated into two antigenically distinct fractions; the shorter A band (minor LPS component) and the longer B band. A partial structures for the core oligosaccharide of several strains of P. aeruginosa were proposed Structural study on the A band component (including the B band) is important because of the possible usefulness as protective immunogens against infections. To this end, mass spectrometry is essential technique at several stages. We will investigate the potential of MALDI-MS and PSD techniques for the core-oligosaccharide samples to get structural information. Of course, GC-MS has been used to provide the information of sugar composition.